Tablet Example

When you use a tablet with Qt applications, QTabletEvents are generated. You need to reimplement the tabletEvent() event handler if you want to handle tablet events. Events are generated when the device used for drawing enters and leaves the proximity of the tablet (i.e., when it is close but not pressed down on it), when a device is pushed down and released from it, and when a device is moved on the tablet.

The information available in QTabletEvent depends on the device used. The tablet in this example has two different devices for drawing: a stylus and an airbrush. For both devices the event contains the position of the device, pressure on the tablet, vertical tilt, and horizontal tilt (i.e, the angle between the device and the perpendicular of the tablet). The airbrush has a finger wheel; the position of this is also available in the tablet event.

In this example we implement a drawing program. You can use the stylus to draw on the tablet as you use a pencil on paper. When you draw with the airbrush you get a spray of paint; the finger wheel is used to change the density of the spray. The pressure and tilt can change the alpha and saturation values of the QColor and the width of the QPen used for drawing.

The example consists of the following:

The MainWindow class inherits QMainWindow and creates the examples menus and connect their slots and signals.

The TabletCanvas class inherits QWidget and receives tablet events. It uses the events to paint on a offscreen pixmap, which it draws onto itself.

The TabletApplication class inherits QApplication. This class handles tablet events that are not sent to tabletEvent(). We will look at this later.

The main() function creates a MainWindow and shows it as a top level window.

MainWindow Class Definition

The MainWindow creates a TabletCanvas and sets it as its center widget.

The QActions let the user select if the tablets pressure and tilt should change the pen width, color alpha component and color saturation. createActions() creates all actions, and createMenus() sets up the menus with the actions. We have one QActionGroup for the actions that alter the alpha channel, color saturation and line width respectively. The action groups are connected to the alphaActionTriggered(), colorSaturationActiontriggered(), and lineWidthActionTriggered() slots, which calls functions in myCanvas.

The TabletCanvas class supports two ways by which the alpha channel of the drawing color can be changed: tablet pressure and tilt. We have one action for each and an action if the alpha channel should not be changed.

createActions() creates all actions and action groups of the example. We look at the creation of one action group and its actions. See the application example if you want a high-level introduction to QActions.

We want the user to be able to choose if the drawing color's alpha component should be changed by the tablet pressure or tilt. We have one action for each choice and an action if the alpha channel is not to be changed, i.e, the color is opaque. We make the actions checkable; the alphaChannelGroup will then ensure that only one of the actions are checked at any time. The triggered() signal is emitted when an action is checked.

The canvas can change the alpha channel, color saturation, and line width of the drawing. We have one enum for each of these; their values decide if it is the tablet pressure or tilt that will alter them. We keep a private variable for each, the alphaChannelType, colorSturationType, and penWidthType, which we provide access functions for.

We draw on a QPixmap with myPen and myBrush using myColor. The saveImage() and loadImage() saves and loads the QPixmap to disk. The pixmap is drawn on the widget in paintEvent(). The pointerType and deviceType keeps the type of pointer, which is either a pen or an eraser, and device currently used on the tablet, which is either a stylus or an airbrush.

The interpretation of events from the tablet is done in tabletEvent(); paintPixmap(), updateBrush(), and brushPattern() are helper functions used by tabletEvent().

We get three kind of events to this function: TabletPress, TabletRelease, and TabletMove, which is generated when a device is pressed down on, leaves, or moves on the tablet. We set the deviceDown to true when a device is pressed down on the tablet; we then know when we should draw when we receive move events. We have implemented the updateBrush() and paintPixmap() helper functions to update myBrush and myPen after the state of alphaChannelType, colorSaturationType, and lineWidthType.

In this function we draw on the pixmap based on the movement of the device. If the device used on the tablet is a stylus we want to draw a line between the positions of the stylus recorded in polyLine. We also assume that this is a reasonable handling of any unknown device, but update the statusbar with a warning so that the user can see that for his tablet he might have to implement special handling. If it is an airbrush we want to draw a circle of points with a point density based on the tangential pressure, which is the position of the finger wheel on the airbrush. We use the Qt::BrushStyle to draw the points as it has styles that draw points with different density; we select the style based on the tangential pressure in brushPattern().

We return a brush style with a point density that increases with the tangential pressure.

In updateBrush() we set the pen and brush used for drawing to match alphaChannelType, lineWidthType, colorSaturationType, and myColor. We will examine the code to set up myBrush and myPen for each of these variables:

We fetch the current drawingcolor's hue, saturation, value, and alpha values. hValue and vValue are set to the horizontal and vertical tilt as a number from 0 to 255. The original values are in degrees from -60 to 60, i.e., 0 equals -60, 127 equals 0, and 255 equals 60 degrees. The angle measured is between the device and the perpendicular of the tablet (see QTabletEvent for an illustration).

The alpha channel of QColor is given as a number between 0 and 255 where 0 is transparent and 255 is opaque. pressure() returns the pressure as a qreal between 0.0 and 1.0. By subtracting 127 from the tilt values and taking the absolute value we get the smallest alpha values (i.e., the color is most transparent) when the pen is perpendicular to the tablet. We select the largest of the vertical and horizontal tilt value.

We finally check wether the pointer is the stylus or the eraser. If it is the eraser, we set the color to the background color of the pixmap an let the pressure decide the pen width, else we set the colors we have set up previously in the function.

We keep a TabletCanvas we send the device type of the events we handle in the event() function to. The TabletEnterProximity and TabletLeaveProximity events are not sendt to the QApplication object, while other tablet events are sendt to the QWidget's event(), which sends them on to tabletEvent(). Since we want to handle these events we have implemented TabletApplication.

We use this function to handle the TabletEnterProximity and TabletLeaveProximity events, which is generated when a device enters and leaves the proximity of the tablet. The intended use of these events is to do work that is dependent on what kind of device is used on the tablet. This way, you don't have to do this work when other events are generated, which is more frequently than the leave and enter proximity events. We call setTabletDevice() in TabletCanvas.

In the main() function we create a MainWinow and display it as a top level window. We use the TabletApplication class. We need to set the canvas after the application is created. We cannot use classes that implement event handling before an QApplication object is instantiated.